This invention relates to a measuring apparatus utilizing the spectrum profile of an incident pulse beam, such as an ultrasonic beam, on the object to be measured. More particularly, it relates to an apparatus having an effective signal processing means to measure and image some characteristics of the object quantitatively by correcting the distorted spectrum profile obtained from the received signal.
Such measuring apparatuses involve radars, sonars, ultrasonic equipment and so on. Hereafter, an ultrasonic device will be described as an example of the present invention, but the invention is applicable to other types of equipment utilizing electromagnetic waves.
The ultrasonic measuring devices are used for various purposes such as medical diagnosis, industrial crack detectors and the like. Generally, an ultrasonic pressure wave beam is generated in a transducer converting electric energy to acoustic energy and is applied to an object to be measured. If the object is completely homogeneous, the beam simply transmits through the object and is subject to attenuation, but, from a practical viewpoint, an object to be measured is heterogeneous and contains various structures of different acoustic characteristics. Accordingly, the beam is reflected at many points inside the object, thus generating multi-reflection. The reflection of the ultrasonic sound in a medium is caused by the difference of acoustic impedance (defined as the product of material density and sound speed) of the adjoining tissues. Hereafter such a boundary will be called a reflecting element. As the applied ultrasonic beam, usually in the form of a pulse, travels through the object, it is affected by the acoustic characteristics of the object such as attenuation, frequency dependency of the attenuation, reflection, diffraction, scattering and the like. As a result, the ultrasonic beam is carrying some acoustic information regarding the internal structure of the object. The signal carried by the transmitted beam or the reflected beam (echo) is received by a receiving transducer and converted into an electric signal which is processed by electronic devices to read the carried information and to display it on a display device such as a cathode ray tube.
In a heterogeneous medium (object) like biological tissue, composed of media of various kinds and structure, the propagating ultrasonic pulse waves have a complicated behavior. A pulse beam is reflected at each of the reflecting elements which are located in the beam path and generates multiple echos. Also, the pulse beam is diffracted or scattered. Thus some of the pulse waves will take the same path and interfere with each other. Moreover, the reflection coefficient is very frequency dependent. These provide serious distortion to the received signal, which make it difficult to quantitatively recover the travelled spectrum of the incident pulse, which reaches the reflecting element and returns back to the detector and the information regarding the object becomes ambiguous. In the case of transmission, some parts of the transmitted beam take different paths due to refractions and diffractions before being finally received by the receiver. This causes pulse overlap and gives quite the same effects as reflection. This is serious problem, and exact recovery of the incident spectrum by eliminating the distortion in the received signal has been desired. Many approaches have been considered for solving this problem.
Hereafter, the term "travelled spectrum" is used to mean the received echo spectrum less the effect of pulse overlap (scallopings) and frequency dependent reflection in the reflection mode, and the transmitted pulse spectrum less the multi-path effect in the transmission mode.